Water heater and method of operating a water heater

ABSTRACT

A water heater that includes a tank for holding water, a heat source, a temperature sensor, a communication module operable to communicate with an external controller remote from the water heater, and a controller including a processor and a computer readable memory storing instructions that, when executed by the processor, cause the controller to operate the water heater. The communication module receives commands from the external controller for further operating the water heater.

BACKGROUND

Utility companies (electric utility now, natural gas utility in thefuture) would like to regulate a water heater's load to avoid peak drawsin power by spreading over time the energy required to heat the water.There are several commands that the utilities would like to use.Commands like shed (or reduce) load, add load, relative pricing (staticor dynamic), or grid guidance. The invention relates to using thesecommands to mange water temperature and user satisfaction while at thesame time shifting the energy to heat the tank to off-peak hours.

SUMMARY

In one embodiment, the invention provides a method of operating a waterheater to be placed at a consumer location and to be in communicationwith an external controller. The water heater includes a tank forholding water, a first heating element extending into the tank, a secondheating element extending into the tank, a first temperature sensorcoupled to the tank, and a second temperature sensor coupled to thetank. The method includes sensing a first temperature with the firsttemperature sensor, determining a first temperature value related to thefirst temperature, sensing a second temperature with the secondtemperature sensor, determining a second temperature value related tothe second temperature, and receiving a command from the externalcontroller. The method further includes, when the received command is afirst command, controlling current to the first heating element based onthe first temperature value traversing a first set point, andcontrolling current to the second heating element based on the secondtemperature value traversing a second set point. The method alsoincludes, when the received command is a second command, controllingcurrent only to the first heating element and not the second heatingelement, the controlling being based on the first temperature value.

In another embodiment, the invention provides another method ofoperating a water heater to be placed at a consumer location and to bein communication with an external controller. The method includessensing a temperature with a temperature sensor, determining atemperature value related to the temperature, and receiving a commandand a ratio from an external controller. The method further includes,when the received command is a first command, providing a plurality ofslot time periods, providing a random number, which is limited by thenumber of the plurality of slot time periods, controlling a heat sourceunder a first strategy for a first group of slot time periods based onthe random number and the ratio, and controlling the heat source under asecond strategy for a second group of slot time periods based on therandom number and the ratio.

In yet another embodiment, the invention provides a water heater forperforming the methods of operation. The water heater includes a tankfor holding water, a first heating element extending into the tank, asecond heating element extending into the tank, a first temperaturesensor thermally coupled to the water, a second temperature sensorthermally coupled to the water, a communication module operable tocommunicate with an external controller remote from the water heater,and a controller including a processor and a computer readable memorystoring instructions that, when executed by the processor, cause thecontroller to operate the water heaters.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically represents an energy management systemincorporating aspects of the invention.

FIG. 2 is a sectional view of a water heater capable of being used inthe system of FIG. 1.

FIG. 3 is a block diagram of portions of the control circuit for thewater heater of FIG. 2.

DETAILED DESCRIPTION

Before any constructions of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other constructions and of being practicedor of being carried out in various ways.

FIG. 1 schematically represents an energy management system 10 with aplurality of consumers 15 receiving electrical energy from one or moresuppliers 20 via a distribution system 25. The one or more suppliers 20may be a public electric utility or a natural gas utility, thedistribution system 25 may be an electrical power grid or a gasdistribution system, and the consumers 15 may be residences, businesses,or energy-usage devices of the residences or businesses.

A grid controller 30 can receive information from one or more of thesuppliers 20, distribution system 25, and consumers 15, and control thedistribution of energy (e.g., electrical energy or natural gas) from thesuppliers 20 to the consumers 15 based on the received information. Thecommunication of the information is via a communication network 35. Thecommunication network 35 may be a network or multiple networks made upof hardware, software, or both, such as the Internet, telephone,Ethernet, analog cellular, digital cellular, short range radio wireless,Zigbee, HomePlug, Wifi, WiMax, broadband over power line, coaxial cable,and the like. The grid controller 30 is typically referred to as a smartgrid controller and the energy management system 10 shown in FIG. 1 istypically referred to as a smart grid. The grid controller 30 istypically one or more computers adapted to and responsible forcoordinating and controlling the smart grid (i.e., commanding thedistribution of electrical energy from the suppliers to the consumersand even the consumer loads).

The description thus far with FIG. 1 is a general description of a smartgrid and is intended to be generic. It is envisioned that theimprovements discussed and claimed herein can be used with many smartgrid arrangements, and the discussed structural arrangements for FIG. 1are not intended to be a limitation on the invention. The gridcontroller 30 is in communication with thousands or even millions ofconsumers and consumer loads 15. The grid controller 30 has verysophisticated databases and analysis and control algorithms that monitorreal time load and capacity information as well as expected patterns andother prediction information such as weather and planned constructioninterruptions. Again, why or how the grid controller 30 controls thesmart grid 10 is inconsequential for the invention. Rather, what isrelevant is that the grid controller issues commands to consumers and/ortheir loads 15. Further information regarding various commands isprovided herein. The grid controller 30 may be owned and operated by thecompany or utility that owns and operates the suppliers 20 and/ordistribution system 25. Alternatively, multiple organizations and/oragencies may divide the work and responsibility of operating variousparts of the energy delivery and load control system.

One common load for a consumer 15 is a storage-type water heater. Anexemplary electrical storage-type water heater 100 incorporating aspectsof the invention is shown in FIG. 2; however, the invention may beapplicable to a gas storage-type water heater. The water heater 100includes a permanently enclosed water tank 105, a shell 110 surroundingthe water tank 105, and foam insulation 115 filling the annular spacebetween the water tank 105 and shell 110. A water inlet line or dip tube120 and a water outlet line 125 enter the top of the water tank 105. Thewater inlet line 120 has an inlet opening 130 for adding cold water nearthe bottom of the water tank 105. The water outlet line 125 has anoutlet opening 135 for withdrawing hot water from near the top of thewater tank 105. Multiple resistance heating elements 140 and 145 extendthrough the wall of the water tank 105. In alternative to the heatingelements 140 and 145, the water heater 100 can include a gas burner or acombination of an electric heating element and a gas burner. Each of theelectric heating element and the gas burner can be referred togenerically as a heat source. The below discussion regarding the waterheater 100 and the operations of the water heater 100 will be directedto a heater having multiple resistance heating elements. However, thediscussion below can be extended to a gas storage-type water heater or amix source storage-type water heater.

Referring again to FIG. 2, multiple temperature sensors 150 and 155 arethermodynamically coupled with the outer wall of water tank 105 forindirectly sensing the temperature of water in the water tank 105. Thetemperature sensors 150 and 155 are connected to element control boxes175 and 180 by electrical wire 160 and 165, respectively. Electric A.C.power is supplied to the heating elements 140 and 145 through line 170.Activation/deactivation of each heating element 140 and 145 iscontrolled, in part, by respective relays that receive control signals.A user interface may be mounted on the outside of the water heater 100to permit communication with a control box and provides securityprotected access for control of the heating elements 140 and 145. Theuser interface may be operable to provide direct or remote control ofthe water heater 100. The control box can control the heating elements140 and 145 by providing a control signal on signal lines V_(I/O). Thecontrol box can be distinct from the user interface and element boxes175 and 180, can be incorporated with the user interface, or can beincorporated in one of the element boxes 175 and 180. It is alsoenvisioned that the control box, the user interface, and the elementboxes can be combined in one unit or box.

For the construction shown in FIG. 2, heating element 140 is located inthe lower portion of the tank 105 and heating element 145 is located inthe upper portion of the tank 105. Communication and control among thesensors 150 and 155 with the control box is accomplished through acommunication link. Control circuitry in the control box might take theform of a programmable device as discussed further below. It isenvisioned that more than two heating elements and/or two temperaturesensors could be installed as part of the water heater 100, if desired.Regardless of the exact control circuitry used, or whether a singlecontrol box or multiple control boxes are implemented, the heatingelements 140 and 145 in FIG. 2 are individually controlled. Moreover,feedback mechanisms employing the temperature sensors 150 and 155 may beused to trigger activation of the heating elements 140 and 145.

As part of the water heater, a mixing valve 185 may be used. The valve185 may be controlled through a communication link V_(I/O) coupled tothe control box. Thus, for example, if overheated water is sensed by atemperature sensor, such as sensor 155, then cold water may beintroduced into the overheated water. Alternatively, the mixing valve185 may be coupled in the output conduit 125 of the water heater 110.

The water heater 100 may include an ambient or room temperature sensor190. The ambient temperature sensor 190 is located external to the waterheater 100, but is located within the surrounding environment of thewater heater 100 and senses the temperature of the surroundingenvironment of the water heater 100. Of course, the water heater 100 mayinclude additional temperature sensors, may include other sensors (e.g.,a water consistency sensor), and may include other electrical components(e.g., a powered anode).

Before proceeding further, uneven heating often occurs in storage-typewater heaters. Uneven heating results in the creation of “stacking” or“stratification” where water that is heated rises to the top of thetank. Generally, warmer water is less dense and, therefore, rises. Thus,the temperature of the water within the tank 105 generally increases inthe positive y-direction with warm water at the bottom and hot water atthe top. As a result, non-uniform temperature strata are formed withinthe tank 105. The stacking effect is exacerbated with the introductionof cold water through the water inlet line 120.

Referring now to FIG. 3, the water heater 100 includes a controller 200electrically connected to the first and second heating elements 140 and145, and the first and second water temperature sensors 150 and 155. Thecontroller 200 is also connected to power supply 205, communicationmodule 210, user interface 215, and drivers 220 and 225. The controller200 may also be connected to other electrical elements of the waterheater 100 such as the ambient temperature sensor 190 and a poweredanode. As discussed above, the various elements shown in FIG. 3 can behoused in, supported by, or coupled to one or more control boxes.

In general terms, the controller 200 receives alternating current (AC)(e.g., 220 Volts AC) from power line 170; modulates or controllablyprovides the voltage to the first and second heating elements 140 and145 via drivers 220 and 225, respectively; and receives first and secondtemperature signals from the first and second temperature sensors 150and 155 to control the heating elements 140 and 145. The controller 200also receives commands and command information from a user via the userinterface 215 and an external controller via the communication module210. The external controller can be the grid controller 30 or a housecontroller (sometimes referred to as a “smart house” controller).

The controller 200 includes combinations of software and hardware. Inone construction, the controller 200 includes a printed circuit board(“PCB”) that is populated with a plurality of electrical and electroniccomponents that provide power, operational control, and protection tothe water heater 100. In some constructions, the PCB includes, forexample, a processing unit 230 (e.g., a microprocessor, amicrocontroller, or another suitable programmable device or combinationof programmable devices), a memory 235, and a bus. The bus connectsvarious components of the PCB, including the memory 235, to theprocessing unit 230. The memory 235 includes, for example, a read-onlymemory (“ROM”), a random access memory (“RAM”), an electrically erasableprogrammable read-only memory (“EEPROM”), a flash memory, a hard disk,or another suitable magnetic, optical, physical, or electronic memorydevice. The processing unit 230 is connected to the memory 235 andexecutes software that is capable of being stored in the RAM (e.g.,during execution), the ROM (e.g., on a generally permanent basis), oranother non-transitory computer readable medium such as another memoryor a disc. Additionally or alternatively, the memory 235 is included inthe processing unit 230. The controller 200 also includes aninput/output (“I/O”) system 240 that includes routines for transferringinformation between components within the controller 200 and othercomponents of the water heater. For example, the I/O system communicateswith the communications module 210 and the user interface 215.

Software included in the implementation of the water heater 100 isstored in the memory 235 of the controller 200. The software includes,for example, firmware, one or more applications, program data, one ormore program modules, and other executable instructions. The controller200 is configured to retrieve from memory and execute, among otherthings, instructions related to the control processes and methodsdescribed herein. For example, the controller 200 is configured toexecute instructions retrieved from the memory 235 for controlling theheating elements 140 and 145 to heat the water to one or more set pointsbased on one or more sensed temperatures of the temperature sensors 150and 155 and further based on one or more commands and information viathe user interface 215 and communication module 210.

The PCB also includes, among other things, a plurality of additionalpassive and active components such as resistors, capacitors, inductors,integrated circuits, converters, and amplifiers. These components arearranged and connected to provide a plurality of electrical functions tothe PCB including, among other things, filtering, signal conditioning,signal converting, or voltage regulation. For descriptive purposes, thePCB and the electrical components populated on the PCB are collectivelyreferred to as the controller 105.

The user interface 215 allows a user to interact with the controller 200to control the operation of the water heater 100. The user interface 215is operably coupled to the controller 200 to control, for example, theset point(s) of the water heater 100. The user interface 215 can includecombination of digital and analog input devices required to inputuser-defined control for the water heater 100. For example, the userinterface 215 can include an electronic-based device having a display, atouch-screen display, a plurality of knobs, dials, switches, buttons,faders, or the like.

The power supply module 205 supplies a plurality of nominal voltage tothe various electrical components of the water heater. The power supplymodule 205 is powered by mains power having nominal line voltagesbetween, for example, 100V and 240V AC and frequencies of approximately50-60 Hz. The control of mains power to the heating elements 140 and 145is via drivers 220 and 225, respectively. The drivers 220 and 225 maysimply be respective relays receiving respective control signals fromthe controller 200. The power supply module 205 is also configured tosupply lower voltages to operate circuits and components of the waterheater 100, such as the controller 200, the communication module 210,and the user interface 215.

The communication module allows the controller 200 to communicate withthe grid controller 30 or the house controller (collectively referred toas an external controller). For example, the controller 200 can receivecommands and command information from the external controller. Anexemplary communication module is a USNAP compatible device providing ademand response (DR) or home area network (HAN) solution for the waterheater 100.

During a first (also referred to herein as “normal” or “full-load”)operation of the water heater 100, both of the heating elements 140 and145 (as well as any additional heating elements not shown) heat thewater as is customary for the water heater 100. For example anddiscussed below, the grid controller 30 can provide commands to thewater heater 100 that influences the operation of the water heater 100.The command can be a direct command to the water heater 100 or via asmart house controller. The first operation of the water heater 100occurs when no command or a normal operation (or full-load) command isprovided to the water heater 100 from the grid control 30. In normaloperation, the water heater 100 can operate as if the water heater 100would operate without being connected to the smart grid.

Before proceeding further, it should be understood that variousadjectives or identifiers, such as normal, full load, reduced load,return load, and add load, are used throughout the description. Theterms are used to better identify an operation of the water heater 100.It should be understood to someone skilled in the art that varioussynonyms can be used to in place of the identifiers used herein.

During normal or full-load operation, the upper and lower heatingelements 145 and 140 are controlled based on the related temperaturesfrom the upper and lower temperature sensors 155 and 150, respectively.If a value related to the sensed upper temperature traverses (i.e., isless than) a user set-point minus an upper differential, then the upperheating element 145 turns on. The upper differential may be zero, but istypically a value greater than zero to prevent significant on/offcycling of the heating element 145. One may also view the resultingtemperature of the user set-point temperature minus an upperdifferential as a first set-point temperature. If the value related tothe sensed upper temperature traverses (i.e., is greater than) the userset-point (which can be a identified as a second set-point) then theupper heating element 145 turns off.

Continuing further during full-load operation, if a value related to thesensed lower temperature traverses (i.e., is less than) the userset-point minus a lower differential, then the lower heating element 140turns on. If the lower temperature traverses (i.e., is greater than) theuser set-point or the upper heating element is on, then the lowerelement turns or is kept off.

As discussed already, the water heater 100, by itself or through a homeenergy system, may participate in a smart grid operation. For example,the home owner may participate in smart grid operation to receive afavorable energy rate from the electric utility. Alternatively, the homeowner may be forced to participate in the smart grid operation or face apenalty.

Regardless, during a smart grid operation, the home owner may receive a“reduce-load” command (as referred to as a “shed-load” command). Afterreceiving the reduce-load command, the controller 200 controls the lowerheating element 140 to turn or be kept off. That is, the “reduce-load”command forces the lower heating element to turn off 140. However,during the reduce-load command, the controller 200 operates the upperheating element 145, such as in normal operation. This way the waterheater 100 continues to provide a limited amount of hot water whilereducing the energy consumption of the water heater 100. That is, thestratification of the water in the storage tank allows a limited amountof hot water that is heated for the user, but the whole tank of water isnot heated since the lower element 140 is not activated. The upperheating element may be referred to as the primary heating element orprimary heat source, and the lower heating element may be referred to asthe secondary heating element or secondary heat source.

Following a reduce-load command, the grid controller 30 may control thereturn of the water heaters 100 (or other loads) on the grid. Forexample, if all water heaters 100 being limited by the grid controller30 return to normal operation at the same time, then a second overuse ofthe grid may occur. The grid controller 30 can communicate a“return-load” command (may also be referred to as an “end-shed-load”command). The return-load command controllably returns a large-number ofwater heaters among the many consumers 15.

One technique for returning a water heater 100 is through a“slot-control” method. In a slot-control method, a time period (e.g.,one hour) is divided into slots (e.g., six slots). Each slot thereforehas a slot time period (e.g., ten minutes when continuing the previousexample). The grid controller 30 can communicate a ratio (e.g., in theform of a percentage or a decimal) to the various water heaters 100. Forexample, the grid controller 30 can communicate a return-load command atfifty percent to the water heater 100. Using this ratio, the waterheater 100 can control the water heater 100 in a normal (or full-load)operation for half of the slots (referred to as the active slots) anduse a reduce-load operation for the other half of the slots (referred toas the inactive slots). In one implementation, the active slots arecontinuous and the inactive slots are continuous within the time period.

The ratio information with the return-load command enables the energy inthe lower portion of the tank 105 to heat over a shorter time. If theratio command was not implemented, then all the tanks would see thereturn-load command at the same time and all heaters 100 would heat thelower portion at the same time causing a spike in power on the grid,therefore a time sequence method is used.

Further, the grid controller 30 will want to control how the activeslots are distributed among the various water heaters 100. For example,if sixty water heaters 100 are being controlled, then the smart griddoes not want all sixty water heaters 100 having their active andinactive slots being at the same time. Rather, the smart grid wants totry to distribute the active and inactive slots evenly. For example, thegrid controller 30 will have ten water heaters 100 to first turn-on inslot one, ten water heaters to first turn-on in slot two, and so on. Oneway this is accomplished is for each water heater 100 to have a randomlyassigned initial active slot. For example, each water heater can have arandom number assigned between, and including, zero and five. Theassigned number (e.g., two) identifies which slot should be the firstactive slot when receiving the return-load command. The random numbermay be assigned during manufacturing, power up, or upon receiving thereturn-load command. Since thousands, possibly millions, of waterheaters 100 are coupled to the smart grid, the utilization of randomnumbers by the water heaters 100 statistically spreads the first activeperiod among the various water heaters 100.

It also envisioned that the number of slots can be communicated fromgrid controller 30. For example, the grid controller 100 can communicatefour slots and seventy-five percent with the return-load command. Thecontroller 200 would then divide the time period into four slot timeperiods and randomly assign a first active slot from zero to three. Iffor example zero is assigned, then the first time slot will start theactive period and then the active period would continue to the thirdtime slot. The fourth time slot would then be the inactive period.

In some implementations, the water heater 100 acts in normal operationfor the active time slots and operates in reduce-load operation for theinactive slots. With this operation, the grid controllably returns thewater heaters 100 from a reduce-load state to a normal state.

The time period for return-load command, in some constructions, isbetween thirty and sixty minutes and the slot periods are between fiveand fifteen minutes. These time periods are used to extend relay life.It is also envisioned that both periods may be varied longer towards theend of the warranty period to extend relay life if the relay cycle timeris approaching the warranty limit. This should prolong the relay life toprevent premature failure of the relays.

A slight variation on the above slotted scheme is using the active timeslots for the return-load command to stagger grid loading, but a relayis allowed to stay on until the temperature is satisfied. This willspread the grid load out somewhat, but not allow complete percentcontrol. The result is to give the user better performance and limit therelay cycles.

Another command that the grid controller 30 can communicate with thewater heater 100 is an “add-load” command. For this command, the waterheater 100 adds a defined temperature (e.g., five degrees) to the setpoint. This results in the smart grid to store available energy in thewater heater 100 by overheating the water. The overheating can beminimal (e.g., five degrees) such that the user does not notice theextra heating or can be larger (e.g., twenty degrees) such that a mixingvalve is used to prevent scalding. Regardless, if the grid is underutilized, the grid can issue the add-load command to allow for availableenergy in the water tanks for later use when the grid is being burdened.

Thus, the invention provides, among other things, a new and useful waterheater and method of operating a water heater. Various features andadvantages of the invention are set forth in the following claims.

The invention claimed is:
 1. A water heater comprising: a tank forholding water; a first heating element extending into the tank; a secondheating element extending into the tank; a first temperature sensorthermally coupled to the water; a second temperature sensor thermallycoupled to the water; a communication module operable to communicatewith an external controller remote from the water heater; and acontroller including a processor and a computer readable memory storinginstructions that, when executed by the processor, cause the controllerto determine a first temperature value related to a first temperaturesensed by the first temperature sensor, determine a second temperaturevalue related to a second temperature sensed by the second temperaturesensor, receive a command and a ratio from the external controller, andwhen the received command is a first command, provide a plurality ofslot time periods, provide a random number, which is limited by thenumber of the plurality of slot time periods, control current to thefirst heating element and the second heating element under a firststrategy for a first group of slot time periods based on the randomnumber and the ratio, and control current to the first heating elementand the second heating element under a second strategy for a secondgroup of slot time periods based on the random number and the ratio. 2.The water heater of claim 1 wherein the first group of slot time periodsand the second group of slot time periods form the plurality of slottime periods.
 3. The water heater of claim 1 wherein the instructions,when executed by the processor, further cause the controller to, whenthe received command is a second command, control current to the firstheating element based on the first temperature value traversing a firstset point, and control current to the second heating element based onthe second temperature value traversing a second set point, and when thereceived command is a third command, control current only to the firstheating element and not the second heating element, the control beingbased on the first temperature value.
 4. The water heater of claim 3wherein the instructions, when executed by the processor, cause thecontroller to control current to the first heating element and thesecond heating element under the first strategy for the first group ofslot time periods by, entering the first group of slot time periods,when in the first group of slot time periods, controlling current to thefirst heating element based on the first temperature value traversing afirst set point, and controlling current to the second heating elementbased on the second temperature value traversing a second point, andwherein the instructions, when executed by the processor, cause thecontroller to control current to the first heating element and thesecond heating element under the second strategy for the second group ofslot time periods by entering the second group of time periods, when inthe second group of time periods, controlling current only to the firstheating element and not the second heating element, the controllingbeing based on the first temperature value.
 5. The water heater of claim4 wherein the second command is a full-load command for allowing thewater heater to provide current using the first heating element and thesecond heating element, wherein the third command is a reduced-loadcommand for allowing the water heater to provide current using only thefirst heating element and not the second heating element, and whereinthe first command is a return-load command for allowing, during thefirst group of time periods, the water heater to provide current usingthe first heating element and the second heating element, and forallowing, during the second group of time periods, the water heater toprovide current using only the first heating element and not the secondheating element.
 6. The water heater of claim 1 wherein theinstructions, when executed by the processor, cause the controller tocontrol current to the first heating element and the second heatingelement under a first strategy for a first group of slot time periodsby, entering the first group of slot time periods, and when in the firstgroup of slot time periods, control current to the first heating elementbased on the first temperature value traversing a first set point, andcontrol current to the second heating element based on the secondtemperature value traversing a second point, and wherein theinstructions, when executed by the processor, cause the controller tocontrol current to the first heating element and the second heatingelement under a second strategy for a second group of slot time periodsby, enter the second group of time periods, and when in the second groupof time periods, control current only to the first heating element andnot the second heating element, the controlling being based on the firsttemperature value.
 7. The water heater of claim 1 wherein the ratiodefines a first length of time for the first group of slot time periodsrelative to a second length of time for the second group of slot timeperiods.
 8. The water heater of claim 1 wherein the random numberdefines a first time slot of the first group of slot time periods. 9.The water heater of claim 1 wherein the random number and the ratiodefines a first time slot of the second group of slot time periods. 10.A method of operating a water heater to be placed at a consumer locationand to be in communication with an external controller, the water heaterincluding a tank for holding water, a first heating element extendinginto the tank, a second heating element extending into the tank, a firsttemperature sensor coupled to the tank, and a second temperature sensorcoupled to the tank, the method comprising: sensing a first temperaturewith the first temperature sensor; determining a first temperature valuerelated to the first temperature; sensing a second temperature with thesecond temperature sensor; determining a second temperature valuerelated to the second temperature; receiving a command and a ratio fromthe external controller; and when the received command is a firstcommand, providing a plurality of slot time periods, providing a randomnumber, which is limited by the number of the plurality of slot timeperiods, controlling current to the first heating element and the secondheating element under a first strategy for a first group of slot timeperiods based on the random number and the ratio, and controllingcurrent to the first heating element and the second heating elementunder a second strategy for a second group of slot time periods based onthe random number and the ratio.
 11. The method of claim 10 wherein thefirst group of slot time periods and the second group of slot timeperiods form the plurality of slot time periods.
 12. The method of claim10 and further comprising, when the received command is a secondcommand, controlling current to the first heating element based on thefirst temperature value traversing a first set point, and controllingcurrent to the second heating element based on the second temperaturevalue traversing a second set point, and when the received command is athird command, controlling current only to the first heating element andnot the second heating element, the controlling being based on the firsttemperature value.
 13. The method of claim 12 wherein when the receivedcommand is the first command, entering the first group of time periods,when in the first group of time periods, the step of controlling currentto the first heating element and the second heating element under thefirst strategy includes controlling current to the first heating elementbased on the first temperature value traversing a first set point, andcontrolling current to the second heating element based on the secondtemperature value traversing a second point, entering the second groupof time periods, and when in the second group of time periods, the stepof controlling current to the first heating element and the secondheating element under the second strategy includes controlling currentonly to the first heating element and not the second heating element,the controlling being based on the first temperature value.
 14. Themethod of claim 13 wherein the second command is a full-load command forallowing the water heater to provide current using the first heatingelement and the second heating element, wherein the third command is areduced-load command for allowing the water heater to provide currentusing only the first heating element and not the second heating element,and wherein the first command is a return-load command for allowing,during the first group of time periods, the water heater to providecurrent using the first heating element and the second heating element,and for allowing, during the second group of time periods, the waterheater to provide current using only the first heating element and notthe second heating element.
 15. The method of claim 10 wherein when thereceived command is the first command, entering the first group of timeperiods, when in the first group of time periods, the step ofcontrolling current to the first heating element and the second heatingelement under the first strategy includes controlling current to thefirst heating element based on the first temperature value traversing afirst set point, and controlling current to the second heating elementbased on the second temperature value traversing a second point,entering the second group of time periods, and when in the second groupof time periods, the step of controlling current to the first heatingelement and the second heating element under the second strategyincludes controlling current only to the first heating element and notthe second heating element, the controlling being based on the firsttemperature value.
 16. The method of claim 10 wherein the ratio definesa first length of time for the first group of slot time periods relativeto a second length of time for the second group of slot time periods.17. The method of claim 10 wherein the random number defines a firsttime slot of the first group of slot time periods.
 18. The method ofclaim 17 wherein the random number and the ratio defines a first timeslot of the second group of slot time periods.
 19. A method of operatinga water heater to be placed at a consumer location and to be incommunication with an external controller, the water heater including atank for holding water, a heat source, and a temperature sensor coupledto the tank, the method comprising: sensing a temperature with thetemperature sensor; determining a temperature value related to thetemperature; receiving a command and a ratio from the externalcontroller; and when the received command is a first command, providinga plurality of slot time periods, providing a random number, which islimited by the number of the plurality of slot time periods, controllingthe heat source under a first strategy for a first group of slot timeperiods based on the random number and the ratio, and controlling theheat source under a second strategy for a second group of slot timeperiods based on the random number and the ratio.
 20. The method ofclaim 19 wherein the heat source includes a plurality of heat sourcesand each of the plurality of heat sources is selected from the groupconsisting of a gas burner, an electric resistance heating element.